1. Field of the Invention
The present invention relates generally to the field of photomultiplier power supplies and in particular to a method and apparatus for providing a photomultiplier power supply having a transformer with multiple secondary windings forming cells that can provide voltage ratios to a photomultiplier element in which the ratios can be adjusted by the method of connection to the cell, by the number of turns in the transformer, or by a combination of both.
2. Summary of the Related Art
Power supplies for the provision of power to a photomultiplier tubes is well known in the art. Photomultipliers are used in a variety of applications, including down hole tools which are deployed in a well bore drilled into the earth. The well bore is typically surrounded by a formation. In the typical down hole tool application, the tool traverses the well bore and the photomultiplier tube is used to determine counts which Typically, when providing a power supply to a photomultiplier tube for operation, a high voltage from 500 to 3000 volts from the power supply is usually applied across the terminals of the photomultiplier tube. The photomultiplier tube terminals, a cathode (K) 201 and anode (P) 202, are provided with a proper voltage gradient set up between the photoelectron focusing electrode (F) 203. A dynode is an electrode in an electron tube that functions to produce secondary emission of electrons. Typically dynodes are provided and depending on tube type, an accelerating electrode is also provided. This voltage gradient can be setup by providing a plurality of independent power supplies 200 as shown in FIG. 1, however, in practice provision of a plurality of power supplies typically not practical.
Thus, in practice, as shown in FIG. 2, the inter stage voltage for each electrode is supplied by providing a network of voltage-dividing resistors 205. In addition, designers will often provide additional Zener diodes 206 connected between the anode and the cathode of the Photomultiplier tube. This typical circuit, as shown in FIG. 2 is commonly referred to as a voltage divider circuit or bleeder circuit. The current Ib, flowing through the voltage divider or bleeder circuits shown in FIGS. 2A and 2B is approximately equal to the applied voltage V, divided by the sum of resistor values as follows:Ib=V(R1+R2+R3+R4+R5+R6+R7).
The Zener diodes 206 (Dz) shown in FIG. 2B are used to maintain the inter stage voltages at constant values for stabilizing the photomultiplier tube operation regardless of the magnitude of the anode current. Capacitors C1, C2, C3 and C4 can be connected in parallel with the Zener diodes serves to minimize noise generated by the Zener diodes. Zener diode noise becomes significant when the current flowing through the Zener diodes is insufficient. In this case, the added capacitance is required as the Zener diode noise can affect the signal-to-noise ratio of the Photomultiplier tube output.
As shown in FIGS. 2A and 2B, the general technique used for voltage divider or bleeder circuits is to ground 208 the photomultiplier tube anode 202 and apply a large negative voltage to the cathode of the photomultiplier tube. This grounding and negative voltage attachment scheme eliminates the potential voltage difference between the external circuit and the photomultiplier tube anode, facilitating the connection of circuits such as ammeters and current-to-voltage conversion operational amplifiers to the Photomultiplier tube circuit terminals. In this Photomultiplier tube anode-grounding scheme, however, one must be careful when bringing a grounded metal holder, housing or magnetic shield case near the bulb of the Photomultiplier tube, or allowing such an element to make contact with the bulb, which can cause electrons in the photomultiplier tube to strike the inner bulb wall. Such an occurrence may possibly produce glass scintillation, resulting in a significant increase in noise and data errors associated therewith.
In a photomultiplier tube anode grounding or cathode grounding scheme in either a DC or pulsed operation, when the light level incident on the photomultiplier tube cathode is increased to raise the photomultiplier tube output current, the relationship between the incident light level on the photomultiplier tube and the photomultiplier tube anode current begins to deviate from an ideal linearity relationship at a certain current levels and eventually, the photomultiplier tube output goes into saturation and is no longer linear.
There are numerous problems associated with all known photomultiplier power supply circuits. These problems are usually encountered in deriving a DC signal output from a photomultiplier tube that is representative of a photomultiplier tube count or the height of the pulses, which is representative of the energy level. Typically counts are determined using a biasing voltage divider network 300 as shown in FIG. 3. The current which actually flows through a bleeder resistor such as R7 302 in the resistor network 304, for example the current 306 flowing across resistor R7, as shown in FIG. 3, equals the difference between the bleeder current Ib 308 and the anode current Ip 306 which flows in the opposite direction through the circuit loop of P−Dy5−R7−P. Similarly, for the other bleeder resistors in the resistor network, the actual current flowing through a resistor R1–Rn is the difference between the bleeder current Ib 308 and the dynode current IDyn 310 flowing in the opposite direction through the particular bleeder resistor. The anode current and dynode current flow acts to reduce the bleeder current and the accompanying loss associated with the inter stage voltage supplied becomes more significant in the latter dynode stages which must handle the progressively larger dynode currents. Although the dynode current includes additional current components flowing in the same direction as the bleeder current Ib, 308 in the current context, they are of insignificant magnitude for the present discussion.
For the most part, a reduction of the bleeder currents can be ignored if the anode output is of sufficiently small magnitude. However, when the incident light level is increased and the resultant anode and dynode currents are increased in magnitude, the voltage distribution for each dynode will vary considerably. Because the overall cathode-to-anode voltage is kept relatively constant by the provision of a high-voltage power supply voltage, the loss of the inter stage voltage at the latter stages can be and is redistributed to the previous stages so that there will be an increase in the inter stage voltage.
The loss of the inter stage voltage by the multiplied electron current appears most significantly between the last dynode and the photomultiplier tube anode, but the voltage applied between the last dynode and the photomultiplier tube anode does not contribute to the secondary emission ratio of the last dynode. Therefore, the shift in the voltage distribution to the earlier stages results in a collective increase in current amplification. If the incident light level is increased further so that the anode current becomes of large magnitude, the secondary-electron collection efficiency of the photomultiplier tube anode degrades as the voltage between the last dynode and the photomultiplier tube anode decreases.
Typically, two techniques are applied to increase the maximum linear output. First, photomultiplier power supply designers have used lower the bleeder resistor values to increase the bleeder current. Secondly, photomultiplier power supply designers use a Zener diode between the last dynode and the anode and if necessary between the next to last and second to last stage as well. If the photomultiplier power supply bleeder resistors are located close to the photomultiplier tube, the heat emanating from their resistance may raise the photomultiplier tube temperature, leading to an increase in the undesirable dark current and can induce possible fluctuations in the output signal of the photomultiplier power tube. Furthermore, since design technique requires a high-voltage power supply with a large capacity, it is inadvisable to increase the bleeder current more than necessary. To solve the above problems in applications where a high linear photomultiplier tube output is required, individual power supplies may be used in place of the bleeder resistors at the last few stages of a photomultiplier tube power source. With the Zener diode technique for photomultiplier tube power source design, if the bleeder current becomes insufficient, undesirable Zener diode noise will be generated from the Zener diode, possibly causing detrimental effects on the linearity and associated accuracy output of the photomultiplier tube. Because of this potential inaccuracy in signal from the photomultiplier tube output, it is essential to increase the bleeder current for the photomultiplier tube to an adequate level and connect a ceramic capacitor having an acceptable frequency response in parallel with the Zener diode for absorbing the possible noise in the circuit.
When a photomultiplier tube is pulse-operated, by way of providing a bleeder circuit, such as the circuit shown in FIG. 2A, the maximum linear output of the photomultiplier tube is limited to a fraction of the bleeder current just as in the case of DC voltage operation. To ameliorate or prevent this problem, decoupling capacitors can be connected to the last few stages. These added decoupling capacitors can also supply the photomultiplier tube with an electric charge during the pulse duration, thereby restraining or mitigating the voltage drop between the last dynode and the anode of the photomultiplier tube, thus resulting in a significant improvement in pulse mode operation linearity. Even with the decoupling capacitors, however, the output of the photomultiplier tube deviates form the desired linear range when the average output current exceeds 1/20th to 1/50th of the bleeder current. In particular, care is required at high counting rates even when the output peak current is low.
As discussed above, typical known photomultiplier power supplies typically utilize resistor ladders to provide voltage to a photomultiplier tube. The resistor ladder is undesirable because of their associated high-power consumption and untenable difficulties in providing some voltage distribution ratios. Moreover, typically known photomultiplier power supply designs require a complex transformer construction, making them difficult to manufacture.
As shown in FIG. 4, a known circuit design is shown in which a Cockcroft-Walton voltage multiplier circuit 400 is shown in which an array of diodes 401 are connected in series. Capacitors 402 are connected in series long each side of the alternate connection points. When a reference voltage V 406 is placed at the input 404, this circuit provides voltage potentials of 2V or 3V and so on at each connection point. Therefore, the power supply circuit shown in FIG. 4 functions much like a conventional resistive bleeder circuit. The Cockcroft-Walton voltage multiplier circuit, however, is inordinately complex. Thus, the circuit of FIG. 4, while known, requires additional parts and is undesirably difficult to manufacture.
Thus, there is a need for a less complex photomultiplier power supply design that provides a lower power consumption voltage ratio conversion circuit that also provides good linearity and is easy to manufacture. Low power consumption is critical in many environments, such as down hole tools, where power can be limited and power conservation can be critical. There is also a need for a photomultiplier power supply design that less sensitive to high temperature environments; such as for use in oil field services operations in a down hole environment such as a wire line or monitoring while drilling applications which subject equipments and power supplies to extreme subterranean temperatures.